XIX 



MONITORS 



The senses which we have been considering in previous 

 chapters tells animals what is happening around them, but 

 it is equally important for animals to know what is going 

 on inside their own bodies. Indeed, often the information 

 about internal conditions helps an animal to know more 

 about the external ones. 



How is it that you can take your handkerchief out of your 

 pocket and blow your nose in the dark? It is not primarily 

 by touch, although that sense is involved in manipulating 

 the handkerchief, for your hand goes straight to your pocket 

 and does not grope over the surface of your coat searching 

 for it by touch. And when you have taken it out you raise 

 it directly and accurately to the nose. Similarly in complete 

 darkness you can put a sweet into your mouth — the one 

 mouth you can never see except as a reflected image in a 

 mirror. 



The senses that enable these feats to be done and monitor 

 the bodily processes are the proprioceptive (or self-perceiv- 

 ing) senses ; most people take them so much for granted that 

 they do not know they possess them. These senses are served 

 by nerve-endings in most parts of the body, especially in the 

 muscles, joints and tendons, and in part of the ear. Those 

 in the muscles, joints and tendons form the muscle sense, and 

 together they tell the position of the limbs and the other 

 parts of the body both in movement and at rest. In the 

 muscles the nerve-endings lie among the cells and are stimu- 

 lated by movements; in the tendons they lie among the 

 fibres and respond when they are stretched ; in the joints 

 they lie in the membranes covering the ends of the bones 

 and are stimulated by contact between the different parts 



of the joint surfaces. The messages sent to the brain by these 



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